The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, a thin layer of air develops between the slider and the rotating disk. When the slider rides on this air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
Traditionally, GMR sensors have been constructed as current in plane (CIP) GMR sensors, wherein current flows through the sensor from one side to the other in a direction parallel with the planes of the layers making up the sensor. More recently, increased attention has been focused on current perpendicular to plane (CPP) GMR sensors. As its name suggests, in a CPP sensor, current flows through the sensor from top to bottom in a direction perpendicular to the planes of the layers making up the sensor.
Another type of magnetoresistive sensor is a tunnel junction sensor (TMR) or tunnel valve. A tunnel valve includes a pinned layer and a free layer, similar to a GMR sensor. However, instead of having a non-magnetic electrically conductive spacer layer between the free and pinned layers, a tunnel valve has a thin dielectric, non-magnetic barrier layer, which can be constructed of for example alumina Al2O3. A tunnel valve operates based on the spin dependent scattering of electrons through the thin barrier layer. When the magnetic moments of the free and the pinned layer are aligned parallel with one another, electrons much more readily pass through the barrier layer than when they are the moments are antiparallel. Therefore, current travels through a tunnel valve in a direction perpendicular to the plane of the layers making up the sensor, similar to a current perpendicular to plane (CPP) GMR.
With reference to FIG. 1A, magnetoresistive sensors have been constructed by a method that includes first depositing the sensor layers 102 as full film layers, on a substrate 104, which can be for example an alumina gap layer, or in the case of a CPP GMR or tunnel valve could be an electrically conductive magnetic material such as NiFe. Then, a full film layer of material that is resistant to chemical mechanical polishing (CMP stop) such as diamond like carbon (DLC) 106 is deposited. A layer 108 of a antireflective coating material (ARC), such as Duramide, is then deposited over the CMP resistant material 106. A mask 110 that includes one or more layers of a photoreactive material such as photoresist is then formed to cover the area where the sensor is desired and exposing other portions.
With continued reference to FIG. 1A, a reactive ion etch 112 is then performed to remove portions of the ARC layer 108 and CMP stop 106 that are not covered by the photoresist mask 110. This process is known in the industry as transferring the image of the photoresist mask 110 onto the underlying mask layers 106, 108. The RIE process used to transfer the image of the photoresist mask onto the underlying layers 106, 108 is chosen to be a RIE process that can readily remove the materials making up the underlying layers 106, 108.
With reference now to FIG. 1B, it can be seen that, since the RIE process used to perform the image transfer preferentially removes the layer 108 at a faster rate than it removes the photoresist layer 110, a bulbous or mushroom shape forms on the mask layers 106, 108, 110.
After the image transfer process has been performed, another material removal process 113, such as ion milling, can be used to remove sensor material, thereby defining the sensor. The ions that remove the sensor material during the ion mill 113 travel in a direction that is mostly normal to the planes of the sensor layers. However, not all of the ions travel in a direction that is perfectly normal to the planes of the sensor. A portion of the ions travel at angles up to plus or minus about 5 degrees. This results in an uneven material removal rate near the mask. The amount of sensor material removed decreases gradually toward the sensor, resulting in a downwardly sloping structure outside of the active area of the sensor. This downward slope is the result of the uneven distribution of ions being angled toward the sensor and able to pass under the overhanging mask structure.
Sensor performance depends, to large extent on the clean and precise definition of the sensor by the ion mill process. For example, the trackwidth of the sensor is determined by this ion milling procedure, so accurate location of the side walls is critical. In addition, accurate definition of the trackwidth depends upon having a sharp sensor edge that is as close to vertical as possible. A sloping sensor side wall results in a poorly defined trackwidth. Furthermore, a clean vertical side wall is necessary for efficient free layer biasing, since the hard bias layers will abut this side wall.
Unfortunately, the bulbous mask structure 110 prevents the clean accurate sensor definition. For example, the bulbous mask 110 results in shadowing during the ion mill process 113. In addition, the bulbous mask shape results in non-uniform deposition of layers such as hard bias layers and lead layers.
The above described construction has been called a CMP lift of process. This is because after forming the sensor, layers of material such as hard magnetic bias material, lead material, and a second layer of CMP resistant material such as DLC are deposited over the sensor and mask, completely covering the mask and filling the areas outside of the sensor area. Since the mask structure is completely covered, a chemical lift off cannot be performed to remove the mask structure. Therefore, a chemical mechanical polish (CMP) is performed to remove the mask structure 108, 110 and the layers covering it. The second layer of CMP resistant material defines the termination of the CMP process and the first layer of CMP resistant material 106 protects the sensor from damage during the CMP.
Another method of constructing a sensor involves the use of a bi-layer mask structure (not shown), the bi-layer includes a purposely formed overhang that allows chemical lift off of the mask structure without the use of CMP. The overhang allows mask liftoff chemicals to enter under the overhanging portion of mask to contact the mask unimpeded by the overlying bias, lead and CMP resistant. Although the use of a bi-layer resist allows removal of the mask without the need for CMP, it can be appreciated that the relatively large overhanging mask structure presents even greater challenges to accurate sensor definition than does the previously described CMP liftoff process, for the similar reason that the overhanging portion of the mask structure allows only a portion of the ions to pass under the overhang during the ion milling operation used to define the sensor.
The push for ever increased data capacity and data rate forces engineers and scientists to find ways to make magnetoresistive sensors ever smaller. For example by decreasing the width of the sensor the trackwidth of the recording system can be decreased allowing more tracks of data to be recorded on a recording medium. Unfortunately, as the size of the sensor decreases, the magnetic characteristics of the layers making up the sensor behave less like well defined thin film structures and become unstable. This is especially problematic with regard to the pinned layer structure. As the track width of the sensor shrinks, the pinned layer becomes less stable. As a result the magnetic moment of the pinned layer is prone to switching direction (amplitude flipping). Such amplitude flipping is a catastrophic event that renders the sensor unusable. This amplitude flipping can easily occur during an event such as an electrostatic discharge (ESD) or during a head disk contact, both of which momentarily increase the temperature of the sensor.
Therefore, there is a strong felt need for a method for defining the track width of a magnetoresistive sensor that overcomes the shadowing problems resulting from the bulbous mask structure described above. In addition, there is a strong felt need for a method for overcoming pinned layer instability problems resulting from decreased trackwidth.